As electronic devices increasingly become portable, advances must be made in energy storage systems to enable such portability. Indeed, it is often the case with current electronic technology that the limiting factor to portability of a given device is the size and the weight of the associated energy storage device. A small energy storage device, such as a battery, may be fabricated for a given electrical device but at the cost of energy capacity. Conversely, a long lasting energy source can be built but it is often too large or too bulky to be comfortably portable. The result is that the energy source is either too heavy or does not last long enough for a particular user's application.
Numerous different battery systems have been proposed for use over the years. Early rechargeable battery systems included lead acid, and nickel cadmium (NiCad), each of which has enjoyed considerable success in the market place. Lead acid batteries are preferred for applications in which ruggedness and durability are required and hence have been the choice of automotive and heavy industrial settings. Conversely, NiCad batteries have been preferred for smaller portable applications. More recently, nickel metal hydride systems (NiMH) have found increasing acceptance for both large and small applications.
Notwithstanding the success of the foregoing battery systems, other new batteries are appearing on the horizon which offer the promise of better capacity, better power density, longer cycle life, and lower weight, as compared with the current state of the art. The first such system to reach the market is the lithium ion battery, which has already found its way into numerous consumer products. Lithium polymer batteries are also receiving considerable attention, though they do not yet have a dominant place in the market.
Lithium batteries in general include a positive electrode fabricated of, for example, a transition metal oxide material, and a negative electrode fabricated of an activated carbon material such as graphite or petroleum coke.
The range of operating voltage for these cells is critical. If the cell is too deeply discharged below a critical lower voltage limit, some electrode materials are irreversibly damaged, reducing future cycle life. On the other hand, if the cell is overcharged beyond a critical upper voltage limit the long-term electrode performance may be compromised, and more immediately, short-circuits and or thermal runaway may occur in the cell. Because the potential violence of cell runaway reactions is commensurate with the amount of energy stored in the cells, the ability to prevent cell failures is mounting in importance as the consumer electronics industry moves toward battery cells of higher and higher energy containing flammable components. It is not unusual to find elaborate--and not inexpensive--electronic circuits to manage the electrical current and voltage cut-offs for the cell. In the ideal case the cell would regulate itself by means of reversible self-switching properties, and independently of external circuitry.
Accordingly, there exists a need for improved approaches to constrain cells to pass current only within the operating voltage in electrochemical cells. It will be appreciated that advances in the ability to control current and voltage offer advantages not just for energy storage cells, but also for other types of electrochemical devices.